In order to obtain a desired voltage depending on the intended use, an electricity storage cell such as a secondary battery or an electric double layer capacitor is plurally provided, wherein the plurality of electricity storage cells are connected in series to form an storage module, and used. In the electricity storage module, along with repetitive charging and discharging, a variation in cell voltage occurs among the respective cells, due to variation in capacity, internal resistance, environmental temperature, self-discharge and the like. The variation occurring in the electricity storage module causes problems such as accelerated progression of degradation, and reduction in utilizable energy. Thus, in the case where a plurality of electricity storage cells are connected in series and used, there is a need for an equalization circuit designed to eliminate a voltage variation among the respective cells.
Heretofore, various types of equalization circuits have been employed. However, many of the types require a large number of switches, causing a problem of an increase in complexity of a circuit configuration. As measures against this problem, there have been proposed two types of equalization circuits capable of operating, respectively, using one transistor and two transistors (i.e., one switch and two switches) (see FIG. 1 and FIG. 2 corresponding, respectively, to the following Patent Document 1, and Japanese Patent Application No. 2012-46569 which is an earlier patent application filed by the same applicant as that of the present invention). These schemes make it possible to significantly reduce the number of required switches, and therefore have an advantage of being able to significantly simplify a circuit configuration. However, the schemes are directed to an “equalization circuit” specialized in voltage equalization, so that it is necessary to separately equip a charge-discharge device for charging and discharging the electricity storage cells. That is, in terms of an electricity storage system, it is necessary to equip a “charge-discharge device” in addition to an “equalization circuit”. Thus, there still remains a need for simplification of the electricity storage system.
In this connection, there has been proposed an equalization function-equipped charger capable of operating using one transistor (i.e., one switch) (see FIG. 3 corresponding to the following Patent Document 2). This scheme is based on an integration of a charger and an equalization circuit, and is therefore capable of charging series-connected electricity storage cells while eliminating a voltage variation among the electricity storage cells. That is, there is no need to equip a charger and an equalization circuit in a separate manner, so that it becomes possible to promote simplification in terms of an electricity storage system. However, this scheme is configured to supply charging electric power to the series-connected electricity storage cells, individually. Thus, each of all components composing a circuit needs to be designed to have a relatively large current capacity, so that the circuit tends to be increased in size and cost. Moreover, it does not have any function of a discharge device. Thus, a discharge device is required separately.